Recently in the field of orthodontics, it has become very desirable to wearers of orthodontic brackets to have visually asthetic brackets that are clear or substantially transparent. In response to this need, brackets made of crystalline materials have been introduced. While these materials are asthetically appealing, the breakage of brackets made of these type materials has become a problem.
It is well known that the surface condition of a crystalline orthodontic bracket has a direct effect on its strength particularily with a single crystal bracket. Machining operations, such as diamond wheel grinding, have been shown to effect the fracture strength of the material, primarily due to the introduction of flaws at or near the surfaces that have been machined. Surface flaws that result from conventional machining processes typically are scratches and cracks. Additionally, it is believed that conventional machining methods result in residual surface stresses which also decrease the fracture strength. It has been suggested in the prior art that certain heat treatments can be conducted prior to machining, such as discussed in U.S. Pat. Nos. 4,595,598; 4,639,218; and 4,681,538. It has also been suggested that single crystal alumina orthodontic bracket can obtain increased fracture toughness by post-heat treatment. However, these heat treatments merely relieve residual stresses. These heat treatments do nothing to mend or heal the cracks or flaws formed at the surface.
Applicants have invented an orthodontic bracket having improved fracture toughness which is obtained by subjecting the orthodontic bracket to an improved post-heat treatment process whereby not only the residual stresses are relieved, but healing of micro surface flaws is accomplished.